Life History and Habits

Fungus gnat larvae usually are in the top 5-8 cm of the growing medium, depending on moisture level. They primarily feed on fungi, algae, and decaying plant matter. However, the larvae can feed on plant roots and leaves resting on the growing medium surface. In homes, adult fungus gnats are usually seen in the vicinity of an infested houseplant. However, adult flies may disperse short distances and often accumulate around window frames. During their seven to 10-day life span, females may lay up to 200 eggs into the cracks and crevices of growing media. Moist-growing media containing high amounts of peat moss are particularly attractive to adult females. 

Nuisance

Adult fungus gnats do not bite, but they are a nuisance when their numbers are high and they can be a pest in greenhouses. Adult fungus gnats tend to be most noticeable during late fall and winter. Decreased day length and cooler temperatures slow plant growth and water usage. If watering practices are not altered, particularly during fall and winter, the growing medium will remain moist, which improves conditions for fungus gnat development. Furthermore, as the growing medium ages or degrades it tends to retain more moisture, which will attract fungus gnat adults.  

Management

• An effective means of detecting fungus gnat larvae is to insert ¼ inch slices of potato into the growing medium. Larvae will migrate to the potato and start feeding within a few days, at which point the potato can be removed and checked for larvae presence.  
• The most important strategy to minimize fungus gnat problems associated with houseplants is to allow the growing medium to dry between watering, especially the top 1 to 2 inches. 
• It is also recommended to re-pot plants, particularly when the growing medium is retaining too much moisture 
• Be sure to remove any containers with an abundance of decaying plant matter such as decayed bulbs and roots, which are a food source for fungus gnat larvae. 
• In greenhouse systems, fungus gnat entry can be prevented by storing potting soil indoors or sanitizing via steam or autoclave before potting plants sensitive to fungus gnats. Populations in the greenhouse can be suppressed by reduction of breeding areas. For example, removing decaying plant matter, soil and standing water from underneath benches.  
• Yellow sticky cards can be used to trap adult fungus gnats and should be placed under the plant canopy or on the edge of containers. 
• When fungus gnat populations are excessive, pesticides can be applied to the growing medium. Usually, multiple applications are required.  
• Fungus gnat larvae may be killed by the microbial insecticide Bacillus thuringiensiss (Bt) when applied as a drench to the growing media. Bt is not generally available through retail outlets, but it is used in commercial greenhouse and large interior environments 
• A biological control option for fungus gnat larvae is applications of insect parasitic nematodes applied as a drench to the growing medium. The nematode species Steinernema feltiae is particularly effective against fungus gnat larvae